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The SchedulerWorkerPoolImpl in TaskScheduler will be changed to have a
dynamic number of SchedulerWorkers, so |workers_| can be read and 
written to from multiple threads. The |idle_workers_stack_lock_|
will additionally cover |workers_| and be renamed to |lock_|.

Bug: 738104
Change-Id: I23a49e18253a4a9a375be790be5a6735cb844323
Reviewed-on: https://chromium-review.googlesource.com/556341
Commit-Queue: Jeffrey He <jeffreyhe@google.com>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Robert Liao <robliao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486074}